Techniques for non-destructively inspecting interior of a subject to be examined using a radiolucent image has become techniques necessary in a medical field, an industrial non-destructive inspection field, and the like. Especially, an X-ray image sensor directly taking a radiolucent image as electronic data has been widely used due to promptness of imaging, reading assistance by image processing, capability for responding to moving images, and others. A device mainly used as this X-ray image sensor is one referred to as an FPD (Flat Panel Detector). In the FPD, a photoelectric converter for converting X-rays into electric charge and a switching element for externally taking signal charge accumulated in the photoelectric converter are disposed in each pixel two-dimensionally arranged. The FPD is produced on a large-area substrate such as glass using a thin-film semiconductor technique. The reason is that since it is difficult to produce a reduced optical system for X-rays unlike visible light, a scale of the FPD needs to be at least the same as a subject to be examined. Therefore, as the switching element disposed in a pixel, a TFT (Thin-Film Transistor) is used.
The FPD is roughly divided into two types based on difference in types of converting X-rays into electric charge. One type is an indirect conversion type that converts X-rays into fluorescence to be converted into charge, and the other type is a direct conversion type that directly converts X-rays into charge.
Patent Literature 1 has disclosed, in FIG. 1, a structure as a conventional example of an FPD of the indirect conversion type. This example has a structure where a phosphor layer is laminated on a formation part of a photodiode and a transistor via an insulating film. Fluorescence is emitted from the phosphor layer by X-ray irradiation and then converted into charge by the photodiode. Further, Patent Literature 1 has disclosed an example, in which the photodiode and the transistor used here is formed with a-Si (amorphous silicon).
Patent Literature 2 has disclosed, in FIG. 1, a structure as a conventional example of an FPD of the direct conversion type. This example has a structure where each pixel including a transistor connected to a photoconductive layer is formed on a substrate. X-rays are absorbed by the photoconductive layer and then directly converted into charge. Further, Patent Literature 2 has disclosed examples using ZnO, CdS, CdSe, and the like as the photoconductive layer. In the FPDs of both types, a signal is output as electric charge, converted into voltage in a signal detection circuit such as an integrator externally disposed, and then digitized.
Over recent years, in a medical field, lower exposure and higher definition have been strongly desired for an X-ray diagnostic apparatus. When X-ray irradiation amount is reduced for lower exposure, signal charge detected by the FPD is also decreased, resulting in S/N ratio degradation. Further, even when a pixel size of the FPD is reduced for higher definition, signal charge is also decreased in accordance with this reduction, resulting in S/N ratio degradation. In other words, to achieve both lower exposure and higher definition, it is necessary to increase S/N ratio of the FPD.
As a technique for increasing S/N ratio of an image sensor, available is a technique referred to as APS (Active Pixel Sensor) being applied to a CMOS image sensor. This APS is a technique having been already proposed in the initial stage of developing solid-state imaging devices, and the contents thereof are described, for example, in Non Patent Literature 1. FIG. 16 illustrates a circuit diagram of a pixel according to the APS technique described in Non Patent Literature 1. The following operation is performed in this technique: during a period when COLUMN PULSE n and LINE PULSE m cause transistors T4 and T5 to be in conduction state, voltage of a diode D1 is output to COMMON OUTPUT being an output wiring, by a transistor T3. In other words, a signal is amplified by the transistor T3 and buffered to be output. Therefore, in an output wiring and the like, noise is hardly mixed, resulting in possibility of obtaining high S/N ratio.
Patent Literature 3 and Patent Literature 4 have disclosed methods as conventional examples of an image sensor applying the abovementioned APS technique to a thin-film semiconductor. The method disclosed in Patent Literature 3 is a method using an a-Si TFT as a transistor for amplifying a signal. And, the method disclosed in Patent Literature 4 is a method using a polycrystalline Si TFT as a transistor for amplifying a signal.